This invention relates to a hydraulic circuit which is applied for crawler type vehicles such as bulldozers, power shovels and the like.
Conventionally, there has been known a hydraulic circuit which comprises a hydraulic source; a reservoir tank connected with the hydraulic source through a first hydraulic passage; a change-over valve connected with the hydraulic source and the reservoir tank through second and third hydraulic passages, respectively; a spool valve connected with the change-over valve through a pair of hydraulic pipes; a hydraulic actuator having a pair of inlet-outlet ports; and a pair of inlet-outlet passages each having one end connected with the spool valve and the other end connected with the inlet-outlet port of said hydraulic actuator; a first relief passage having one end connected with one of the inlet-outlet passages and the other end connected with the other of the inlet-outlet passages and provided with a first relief valve relieving a hydraulic fluid in one of the inlet-outlet passages to the other of the inlet-outlet passages when the hydraulic fluid in one of the inlet-outlet passages is raised to a predetermined relief pressure; a second relief passage having one end connected with the other of the inlet-outlet passages and the other end connected with the one of the inlet-outlet passages and provided with a second relief valve relieving a hydraulic fluid in the other of the inlet-outlet passages to the one of the inlet-outlet passages when the hydraulic fluid in the other of the inlet-outlet passages raised to a predetermined relief pressure; the spool valve including a pair of connecting passages each connecting the hydraulic pipe and the inlet-outlet passage and having a main check valve to allow a hydraulic fluid to flow from the hydraulic pipe to the inlet-outlet passage; a pair of spool passages each having one end connected with the connecting passage between the main check valve and the hydraulic pipe and the other end connected with the connecting passage between the main check valve and the inlet-outlet passage; a valve spool axially movable to open and close the spool passages; a pair of compression coil springs urging the valve spool to be brought into a null position; a pair of pilot passages each having one end connected with the connecting passage between the main check valve and the hydraulic pipe to bring the connecting passage into communication with the valve spool so as to axially move the valve spool against the compression coil spring; and a check valve provided on each of the pilot passage to allow the hydraulic fluid from the connecting passage to the valve spool. The conventional hydraulic circuit, however, requires two relief valves and therefore becomes not only complicated and extremely expensive but also bulky in size. Although it may be considered to omit two relief valves and two relief passages as provided in the conventional hydraulic circuit for the purpose of overcoming the aforementioned drawbacks, there is a possibility to damage the hydraulic circuit when a surge pressure occurs in the inlet-outlet passage. In order to avoid the problems it may be also considered to provide a throttle in the pilot passage in parallel with the check valve to slowly axially move the valve spool for opening and closing action of the valve spool. In this case, however, the change-over valve is actuated to instantly close the hydraulic fluid returned from the hydraulic actuator. So that a high pressure is generated in the circuit between the actuator and the valve spool.